1. Field of the Invention
The invention relates generally to an electronically monitored mechanical pipette. More specifically, the invention relates to a transducer assembly for monitoring the position of a volume delivery adjustment mechanism of the pipette, and for signalling the electronic system of the pipette of the changing volume delivery setting.
2. Prior Art
Mechanically operated micropipettes are well known in the art as exemplified by U.S. Pat. No. 4,909,991 to Oshikubo. In such prior art devices, the volume of liquid to be dispensed by the pipette is generally indicated to the operator by means of a mechanical display. The display commonly consists of a set of rotary drums driven by a gear mechanism attached to the actuating shaft of the pipette, such that rotation of the actuating shaft causes the drums to rotate to display a new setting. However, due to unavoidable mechanical wear and tear on pipettes, the amount of fluid actually being delivered by a pipette may not actually correspond to the volume being indicated by the mechanical display. Further, accuracy may degrade over time as the actuating elements, such as the shaft, gears, and rotary drum, wear out.
Electrically driven pipettes are also well known in the art as exemplified by U.S. Pat. No. 4,905,526 to Magnussen, Jr. et al. This type of instrument commonly includes an electronic display for displaying the volume of fluid to be dispensed by the pipette, and an actuator generally comprising an electric drive mechanism, such as a stepper motor. The stepper motor generally drives a rotor, which is attached by a threaded screw to an actuator shaft, the threaded screw changes the rotational motion of the motor into linear motion of the actuator shaft. The shaft thereafter drives a piston to displace fluid for pipetting. Although electrically operated pipettes have some advantages over mechanically operated pipettes, they nevertheless suffer from several drawbacks. Mainly, the enlarged size of an electrically operated pipette, due to the need to accommodate the electric driving mechanism, and the added electronic hardware, make the device very difficult to handle for the operator. Further, monitoring the position of the electric motor for purposes of ensuring desired fluid delivery volume can lead to less than desired delivery accuracy.
Electrically monitored mechanical pipettes are also known in the art as exemplified by U.S. Pat. No. 4,567,780 to Oppenlander et al. This type of instrument generally includes a plunger having an adjustable stroke length which is generally adjusted by rotating the plunger itself. The electrical monitoring system monitors plunger rotation and electronically displays the volume delivery setting corresponding to the plunger position. The device continuously monitors the plunger position and volume delivery setting of the pipette by means of a potentiometer. Although this device overcomes several of the disadvantages of mechanical and electrical pipettes, it nevertheless fails to completely resolve the problem of high power demands during operation. Further, the use of a potentiometer to monitor the position of the plunger is sometimes not desirable.
Electrically driven pipettes which include a transducer assembly are also well known in the art as exemplified by U.S. Pat. No. 4,821,586 to Scordato et al. This instrument uses a Hall-effect transducer to indicate when the volume delivery adjustment mechanism thereof is in its "home" position, and therefore ready to be set to a desired volume delivery setting. However, the volume delivery setting is calculated based on the number of pulses applied to the windings of an actuation motor, which in turn determines the number of steps a threaded element rotates through a set of threads of known pitch. This indicates the distance the plunger moves longitudinally from the "home" position, thus determining the stroke of the piston and the volume of fluid which will be aspirated into the tip of the pipette. Although the electrically driven pipette uses a Hall-effect switch to assist in positioning the volume delivery adjustment mechanism, it nevertheless suffers from several drawbacks. Mainly, the Hall-effect transducer is used only as a "switch" to indicate a "home" position from which a volume delivery setting can be made, instead of directly monitoring the entire range of movement of the volume delivery adjustment mechanism and thereby directly indicating all positions of the mechanism to the electronic assembly of the unit. Therefore, fluid delivery setting cannot be determined directly from the output of the Hall-effect transducer.